In the prior art, ultrasonic motors are known as actuators. One of the ultrasonic motors is a motor using surface acoustic wave (SAW), namely a surface acoustic wave motor. The surface acoustic wave is, for example, a wave of oscillation (Rayleigh wave) which is based on an elliptic motion generated in the surface of an elastic body, and which propagates on the surface of the elastic body. The driving force of the motor is a frictional force which is output based on this elliptic motion. The surface acoustic wave motor is expected for use as a small size motor having excellent operational characteristics of high speed, high response and high thrust. For example, a moving speed of 1 m/sec has been achieved as its characteristics. Further, a demonstration has been made that it can produce a thrust equal to or greater than its self-gravity.
For example, a surface acoustic wave motor is comprised of: a surface acoustic wave substrate as a stator; interdigital electrodes (IDT: Interdigital Transducer) for generating a surface acoustic wave on its surface; a slider placed on the surface acoustic wave substrate for movement; pressure applying means for contacting the slider to the surface acoustic wave substrate at a predetermined pressure to obtain a frictional force (refer to e.g. Japanese Laid-open Patent Publication Hei 09-233865).
Further, an energy recovery type surface acoustic wave motor is known, in which in order to generate a surface acoustic wave in a surface acoustic wave motor, interdigital electrodes provided at an end of a surface acoustic wave substrate are used to supply energy, while the energy of the surface acoustic wave unused for driving is recovered at the other end for reuse (refer to e.g. Japanese Laid-open Patent Publication Hei 11-146665).
Referring to FIGS. 25A and 25B, a conventional energy recovery type surface acoustic wave motor will be described. This surface acoustic wave motor is a linear motor comprising a surface acoustic wave substrate 2 as a stator, and a slider 3 driven on the surface S for linear movement. On the surface S of the surface acoustic wave substrate 2 are provided an energy-complementary interdigital electrode 4 composed of interdigital electrodes 4a and 4b, and a pair of interdigital electrodes 5 placed distant from each other and sandwiching the interdigital electrode 4 therebetween.
The surface acoustic wave substrate 2 is formed of a piezoelectric material having a thickness of about 1 mm. The interdigital electrodes 4 and 5 are formed by patterning a thin film conductor into a shape of comb teeth meshing with each other. The interdigital electrodes 5 recover energy of the surface acoustic wave, and at the same time supply energy to generate a surface acoustic wave. The slider 3 is placed in a moving section 21 provided between the interdigital electrode 4 and the interdigital electrode 5. The slider 3 is being pressed onto the surface S by applied pressure N from pressure applying means 30.
In the above-mentioned state, when a high frequency (MHz range) voltage of a predetermined frequency ω is applied to the interdigital electrode 4b from an external power supply V1 with v1=v0·cos ωt, and to the interdigital electrode 4a from an external power supply V2 with v2=v0 sin ωt, then the interdigital electrodes 4a and 4b convert the electrical energy to mechanical energy of wave, thereby generating a surface acoustic wave W which travels on the surface S rightward in the drawing. Note that the moving direction of a point on the surface S based on the elliptic motion of the surface acoustic wave W is opposite to the direction of travel of the surface acoustic wave W. Frictional force caused by the elliptic motion of the surface acoustic wave W traveling rightward operates the slider 3 so that the slider 3 is driven in a direction (leftward in the drawing) opposite to the direction of travel of the surface acoustic wave W. The surface acoustic wave W passes to the right side of the slider 3 while losing energy to drive the slider 3, and becomes a surface acoustic wave w which further travels rightward.
The interdigital electrode 5 on the right receives and converts mechanical energy from the surface acoustic wave w to electrical energy, performing so-called mechanical to electrical conversion. The energy recovered as electrical energy is sent to the interdigital electrode 5 on the left which is electrically connected via wiring 7. The interdigital electrode 5 on the left performs electromechanical conversion to convert the electrical energy to mechanical energy so as to supply energy to the surface S, contributing to the generation of surface acoustic wave W. This surface acoustic wave motor uses the pair of interdigital electrodes 5 to circulate energy (recover and supply energy), and uses the interdigital electrode 4 to complement the consumption of energy, so as to operate using less energy than not using the interdigital electrodes 5.
However, the energy recovery type surface acoustic wave motors disclosed in the above-mentioned FIGS. 25A and 25B and Japanese Laid-open Patent Publication Hei 11-146665 have problems as described below. This will be described with reference to FIG. 26 and FIG. 27A to FIG. 30B. FIG. 26 shows an enlarged view of a portion where the slider 3 contacts the surface S, in which amplitudes of surface acoustic waves W, w1 and w2 generated on the surface S are shown enlarged. At the portion where the slider 3 contacts the surface S of the surface acoustic wave substrate 2, a surface acoustic wave w2 with a change in phase from the original surface acoustic wave W by a phase difference ΔX is generated.
Thus, the surface acoustic wave w having passed the slider 3 contains both the surface acoustic wave w2 with a change in phase and a surface acoustic wave w1 which is in phase with the surface acoustic wave W. For this reason, the surface acoustic wave w having arrived at the interdigital electrode 5 on the right varies in phase along a width direction perpendicular to the direction of travel thereof, so that when the interdigital electrode 5 converts mechanical energy to electrical energy, the waves partially eliminate each other, causing an energy loss. The reason why the surface acoustic wave w is generated on the surface S in a mixed state of the surface acoustic waves w1 and w2 is because the slider has a width f smaller than a width g of the surface acoustic wave formed by the interdigital electrodes 4 and 5 as shown in FIG. 25A.
The generation of the energy loss due to the presence of the above-mentioned surface acoustic wave w2 is further described. First, energy recovery efficiency will be described. When the surface acoustic waves w1 and w2 have no phase difference, energies e1=e(w1) and e2=e(w2) of the respective waves w1 and w2, and energy e=e(w) of wave recovered from the total wave w are expressed by FIGS. 27A and 27B. Here, e(x) represents an operation to calculate energy of wave. Similarly, when the surface acoustic waves w1 and w2 have a phase difference, energies e1 and e2 of the respective waves w1 and w2, and energy eα=e(w) of wave recovered from the total wave w are expressed by FIGS. 28A and 28B.
If the surface acoustic wave w contains a phase difference as in the latter, a phase difference Δt1 is generated between the energies e1 and e2 corresponding to the phase difference ΔX in FIG. 26. Due to the influence of this phase difference Δt1, the energy ea is lower than the energy e(eα<e), causing a lower energy recovery efficiency. Furthermore, a phase difference Δt2 is generated in the energy ea, although the phase of the energy e is in phase with the phase of the complementary energy supplied from the interdigital electrode 4.
Next, energy supply efficiency will be described. If a complementary energy E0 supplied from the interdigital electrode 4 is in phase with the circulating energy e supplied from the interdigital electrode 5 on the left, these energies E0 and e, and energy E1=e(W(E0+e)) of a wave W generated by these energies are expressed by FIGS. 29A and 29B. Here, W(x) represents an operation to generate a wave. Similarly, when there is a phase difference Δt2 as described above, then a complementary energy E0, a circulating energy eα supplied with the phase difference Δt2, and energy Eβ=e(W(E0+eα)) of a wave W generated by the supply of these energies E0 and eα are expressed by FIGS. 30A and 30B.
If the complementary energy E0 is in phase with the circulating energy e, they do not badly influence each other. However, if not in phase, the energy Eβ is lower than the energy E1(Eα<E1) (sic, correctly: Eβ<E1), causing a lower energy supply efficiency. Furthermore, it occurs that the phase of the energy Eβ has a phase difference Δt3 to the energy E1.
As described above, due to the phase difference ΔX generated by the contact of the slider 3 to the surface S of the surface acoustic wave substrate 2, it occurs that the circulating energy ea and the energy Eβ of the generated surface acoustic wave W are reduced, and the presence of the phase differences Δt1, Δt2 and Δt3 causes energy loss and degradation in driving characteristics. Further, these phase differences are caused not only by the contact of the slider 3 to the surface S, but also by e.g. ambient temperature change which causes a change in the characteristics of the surface acoustic wave substrate 2, and a deviation of an interdigital electrode pattern provided on the surface S from a design value as well. Thus, a conventional surface acoustic wave motor still has a limitation in the reduction in the power to drive the slider 3.
The present invention is to solve the above-described problems, and its object is to provide an energy recovery type surface acoustic wave motor which adjusts the phase change at the time of energy recovery and supply so as to achieve an increase in the energy efficiency.